In recent years, mobile phones or PDAs (Personal Digital Assistants) have been widespread. Also, the specifications in which a compact digital still camera unit or a compact digital video unit is incorporated in the mobile phone or the PDA have been generalized. In a digital apparatus such as the mobile phone or the PDA, a small-sized image sensor with a low pixel number, as compared with an independent product such as a digital still camera, and an imaging lens device provided with a single focus optical system constituted of one to three plastic lens elements are generally used, in view of severe constraints regarding the size or cost of the digital apparatus.
Since the magnification of the single focus optical system is substantially equivalent to visual magnification, an object to be photographed is limited to the one located near a photographer. Under the current rapid development of high-pixel, high-resolution image sensors, there is a demand for a compact zoom optical system that is compatible with a high-pixel image sensor, and is loadable in a mobile phone or a like device capable of photographing a subject remotely away from a photographer.
For instance, patent document 1 proposes an arrangement directed to a three-component zoom optical system of negative-positive-positive arrangement which is constituted of a first lens group having a negative optical power, a second lens group having a positive optical power, and a third lens group having a positive optical power in this order from the object side, wherein the total thickness of the optical system is reduced when a lens barrel is collapsed. Also, patent document 2 discloses a four-component zoom optical system of negative-positive-positive-positive arrangement which is constituted of lens groups having a negative optical power, a positive optical power, a positive optical power, and a positive optical power in this order from the object side, wherein productivity of an aspherical negative lens element in the first lens group is improved by properly selecting a glass material for the aspherical negative lens element.
It is difficult to employ the lens barrel collapsible structure as recited in patent document 1 to a mobile phone or a like device, because an impact resistance required for the mobile phone or a like device is high. Accordingly, the optical system proposed in patent document 1 has an unduly long entire length in use. Also, the second lens group is constituted of three or more lens elements, and the total number of lens elements is as large as six to eight. Therefore, miniaturization has not been completely accomplished in the optical system disclosed in patent document 1. In the zoom optical system recited in patent document 2, the power of the second lens group is weak, and the optical system is not compact because of a large moving amount. In addition to these drawbacks, the number of lens elements is as large as seven. In light of these drawbacks, it is difficult to mount the zoom optical system recited in patent document 2 in a mobile phone or a like device.
The zoom optical systems in patent documents 1 and 2 employ a negative dominant optical system, in which the first lens group closest to the object side has a negative optical power. In the negative dominant optical system, the second lens group primarily serving as an element for zooming is required to have an extremely strong optical power in microminiaturizing the optical system. In this case, particularly at a telephoto end, magnification chromatic aberration resulting from an increase in optical power of the second lens group is unduly increased, which may lower the contrast in the periphery of a captured image, and resultantly cause image degradation.
patent document 1: Japanese Unexamined Patent Publication No. 2002-48975
patent document 2: Japanese Unexamined Patent Publication No. 2002-365543